Or the card is being saturated in such a way that it's not heating up as much. Which is a strange circumstance, but if the memory buffer is topped out, usage generally maxes out as well, but it's not actually 'processing' as fast, because it can't or it doesn't have to anymore (distance textures)

True, but there's not a lot of activity on a desktop -- so while there are more pixels to render and thus more memory used, the speed required would likely not go up. And as LarkStarr suggests, if the memory becomes the limiting factor the processor will slow down. The switch to a higher resolution monitor and to desktop usage could well have saturated the memory and left the processor waiting.